I'm not sure if we are getting to the issue here. I have the problem whether I fly over water or not. It seems that the issue is negative altitude. When I start my flight I am above the beach by 12m or so. If I head down to the sand then the height from my starting point can vary from flight to flight. I know this because when I bring the drone back to the launch point the altitude can read +6m or 5m or something else, not zero. The reason why this is a worry is that I do fly over water and I can't trust the altitude to be correct.